In the field of optical telecommunications, wavelength division multiplexing (WDM), in which optical signals at a plurality of wavelengths are transmitted over one optical fiber, has been used for communicating a large amount of information. According to the 100 G-bps Ethernet (100 GE) standard, four optical signals each of which is capable of communicating information at 25.8 Gbps are communicated over one optical fiber, but some communication standards over 100 GE have recently been developed. For example, the 400-Gbps Ethernet (400 GE) uses the 4-level pulse amplitude modulation (PAM4) WDM as a standard so that eight optical signals each of which is capable of communicating information at 25.8 Gbps are transmitted over one optical fiber.
Furthermore, also known is a technology for reducing the size of an optical transmitter including laser diodes that output optical signals, a driving circuit that drives the laser diodes, and an optical multiplexer that multiplexes the optical signals output from the respective laser diodes, to output an optical transmission signal (see U.S. Patent Application Publication No. 2016/147017, U.S. Patent Application Publication No. 2016/269808, and U.S. Patent Application Publication No. 2015/0300614, for example). Furthermore, also known is a technology for reducing the size of an optical receiver including an optical demultiplexer that demultiplexes an optical reception signal, photodiodes that convert the demultiplexed optical signals into electrical signals, and a transimpedance amplifier that amplifies the converted electrical signals (see U.S. Patent Application Publication No. 2014/346323, U.S. Patent Application Publication No. 2016/0147017, and Japanese Laid-open Patent Publication No. 2016-100360, for example).
In the field of optical telecommunications, there are demands for higher communication rates, and size reductions in optical transmitters and optical receivers.